1. Field of the Invention
The present invention relates to a brush charger and an image forming apparatus, and more particularly to a brush charger for static-charging the surface of a photosensitive member evenly to a specified potential before forming an image on the photosensitive member by an electrophotographic method, and an image forming apparatus provided with the brush charger.
2. Description of Related Art
While electrophotographic full-color image forming apparatuses have been propagated, demand for higher picture quality has been increased. In order to form an image of high quality, it is especially important to static-charge a photosensitive member evenly. Conventionally, a corona discharger is generally used for the purpose. However, a corona discharger charges a photosensitive member by a non-contact method, and therefore, it generates ozone which is harmful to human beings. In order to reduce the amount of ozone which is generated at the time of charging a photosensitive member, a brush charger which charges a photosensitive member by rubbing the photosensitive member with a brush of conductive fibers has been suggested. The following prior arts disclose such brush chargers.
Reference 1 (U.S. Pat. No. 5,479,244) discloses a brush charger which has a brush of synthetic resin semiconductive fibers and teaches that the brush has the following specifications: the fibers are 1.5 to 15 deniers in thickness; the fibers are 2 to 10 mm in length; and the fibers are planted at a density within a range from 50,000 to 500,000 filaments/inch2. However, the fibers are planted on a plane, and the brush charger is a fixed type. The reference 1 does not teach that a roller type brush is rotated.
Reference 2 (U.S. Pat. No. 5,508,788) discloses a fixed type brush of conductive fibers that the fibers are planted at a density within a range from 10,000 to 400,000 filaments/inch2. The reference 2 also discloses a rotary type brush of conductive fibers that the fibers are planted at a density of 100,000 filaments/inch2.
Reference 3 (Japanese Patent Laid-Open Publication No. 2001-242688) discloses an image forming apparatus which adopts a brush charging method in which a superimposed voltage of a DC voltage and an AC voltage is applied for charging of a photosensitive member and adopts a non-contact developing method in which an AC voltage is applied for development of an electrostatic latent image. However, there are no detailed descriptions about the brush.
According to the brush charging method disclosed by the prior arts above, charging of a photosensitive member is carried out by discharge from each of the conductive fibers, and it is difficult to charge the photosensitive member evenly. Consequently, in an image forming apparatus provided with such a conventional brush charger, it has been impossible to obtain full-color images of satisfying quality.